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Horizontal Transfer of Diatomaceous Earth and Botanical
Insecticides in the Common Bed Bug,
Cimex lectularius
L.; Hemiptera: Cimicidae
Yasmin Akhtar, Murray B. Isman*
Faculty of Land and Food Systems, University of British Columbia, Vancouver, BC, Canada
Abstract
Background:
Horizontal transfer of insecticide occurs when insects contact or ingest an insecticide, return to an
aggregation or a nest, and transfer the insecticide to other conspecific insects through contact. This phenomenon has been
reported in a number of insects including social insects, however it has not been reported in bed bugs. Since horizontal
transfer can facilitate the spread of insecticide into hard to reach spaces, it could contribute greatly to the management of
these public health pests.
Methodology/Results:
To demonstrate horizontal transfer of diatomaceous earth and botanical insecticides in C. lectularius,
an exposed (donor) bed bug, following a 10-minute acquisition period, was placed with unexposed (recipient) bed bugs.
Mortality data clearly demonstrates that diatomaceous earth (DE 51) was actively transferred from a single exposed bug to
unexposed bugs in a concentration dependent manner. LC
50
values varied from 24.4 mg at 48 h to 5.1 mg at 216 h when a
single exposed bed bug was placed with 5 unexposed bed bugs. LT
50
values also exhibited a concentration response. LT
50
values varied from 1.8 days to 8.4 days when a ‘donor’ bug exposed to 20 and 5 mg of dust respectively was placed with 5
‘recipient’ bugs. Dust was also actively transferred from adult bed bugs to the nymphs. In addition we observed horizontal
transfer of botanical insecticides including neem, ryania, and rotenone to varying degrees.
Conclusion/Significance:
Our data clearly demonstrate horizontal transfer of diatomaceous earth and botanical insecticides
in the common bed bug, C. lectularius. Use of a fluorescent dust provided visual confirmation that contaminated bed bugs
transfer dust to untreated bed bugs in harborage. This result is important because bedbugs live in hard-to-reach places and
interaction between conspecifics can be exploited for delivery and dissemination of management products directed at this
public health pest.
Citation: Akhtar Y, Isman MB (2013) Horizontal Transfer of Diatomaceous Earth and Botanical Insecticides in the Common Bed Bug, Cimex lectularius L.;
Hemiptera: Cimicidae. PLoS ONE 8(9): e75626. doi:10.1371/journal.pone.0075626
Editor: Joseph Clifton Dickens, United States Department of Agriculture, Beltsville Agricultural Research Center, United States of America
Received June 12, 2013; Accepted August 20, 2013; Published September 25, 2013
Copyright: ß2013 Akhtar, Isman. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: The authors have no support or funding to report.
Competing Interests: RJP Textile Ltd. and SemiosBIO Technologies Inc. have provided funding for bed bug research in the laboratory, but the research
described in the manuscript is completely outside of the scope of the research funded by those companies. This does not alter the authors’ adherence to all the
PLOS ONE policies on sharing data and materials.
* E-mail: murray.isman@ubc.ca
Introduction
Horizontal transfer of insecticides occurs through the transfer of
the active ingredients among individuals within an insect
population through contact. The phenomenon occurs when the
most active members of a colony, often foraging adults, become
exposed to an insecticide residue, which is subsequently trans-
ferred to unexposed members of the colony upon returning to the
nest. Several mechanisms have been described for this transfer
including mutual grooming [1] trophallaxis [2] necrophagy [3]
coprophagy [4] and emetophagy [5].
Horizontal transfer of pathogens (autodissemination) and
insecticides have been reported in several insect orders, including
Blattodea [6] Lepidoptera [7] Coleoptera [8], Diptera [9], [10]
and social insects, viz. Isoptera and Hymenoptera [1], [11].
However, studies of horizontal transfer of insecticides in bed bugs
are lacking.
Bed bugs (Cimex lectularius L.; Hemiptera: Cimicidae) have re-
emerged as important public health pests in the past decade, with
increasing intensity of urban infestations in North America,
western Europe, Japan and Australia [12], [13], [14]. The exact
cause of this resurgence is unclear, but may be a consequence of (i)
the development of resistance in bed bugs [15], [16], [17] to
commonly used domestic insecticides, (ii) increased human
movement – both travel and migration, (iii) decreased public
awareness, and (iv) global warming. Although, the consumer
market is currently flooded with products of dubious composition
and efficacy, the search for new active ingredients and innovative
delivery tools continues to provide effective means of dealing with
bed bugs, one of the most economically and medically important
pests of the urban environment.
Although, the majority of bed bug control methods to date rely
on direct application of insecticides [18] information regarding
secondary mortality due to transfer of insecticide from exposed to
unexposed individuals within the target population is lacking.
PLOS ONE | www.plosone.org 1 September 2013 | Volume 8 | Issue 9 | e75626
Contact insecticides (viz. Sterifab
TM
, Bedlam
TM
) lacking residual
activity against bed bugs require repeated applications to ensure
they reach bed bugs, which remain hidden in crevices except when
seeking and feeding on hosts. Subsequent applications also are
needed to kill nymphs emerging from eggs, or bed bugs
reintroduced into the habitat [19].
Since there are no reports on horizontal transfer of dusts or
other insecticides in bed bugs, the main objective of our study was
to investigate whether adults of C. lectularius exposed to diatoma-
ceous earth and botanical insecticides could transfer them to
unexposed members (adults and nymphs) of a population. A more
thorough understanding of the influence of insecticides on bed
bugs through horizontal transfer could form the basis for designing
effective control strategies against bed bugs.
We have used a specific diatomaceous earth, DE 51, in our
experiments owing to its strong toxic effects against the bed bug, C.
lectularius, in laboratory bioassays (24-h LC
50
and LC
95
val-
ues = 0.24 and 0.95 mg respectively) through direct contact with
the dust (Akhtar and Isman, unpublished data).
Materials and Methods
Samples (Dust and insecticides)
Diatomaceous earth, DE 51, produced by EP Minerals LLC,
Reno, NV, was obtained from JP Textile Ltd., Vancouver, BC,
and a luminous (fluorescent) powder kit #1162A was purchased
from BioQuip Products Inc., Rancho Dominguez, CA.
A refined seed extract of Azadirachta indica ( = neem) (containing
30% azadirachtin) was provided by Fortune Biotech (Bangalore,
India). Rotenone dust was purchased from Later Chemicals Ltd.
(Richmond, BC. Canada) and ryania dust was a gift from Dr. Alan
Knight (USDA ARS, Wenatchee, WA, USA).
Study taxa
Bed sheets heavily infested with bed bugs of all stages were field-
collected from low-income, multiple-dwelling buildings in urban
Vancouver, BC. To the best of our knowledge, no insecticide had
been applied to the premises. Bed bugs were transferred to glass
jars using forceps. Up to 100 individuals were kept in a single glass
container, under a 12:12 LD photoperiod at 2462uC and 65%
relative humidity in the Insect Toxicology Laboratory of the
University of British Columbia, Vancouver, BC. Insects were used
in the experiments within 48 h of collection.
Transfer trials
General procedure (Figure 1): Samples were weighed in small
plastic Petri dishes (Gelmon SciencesH; 5.0 cm diameter) lined
with filter paper (Fisher ScientificH; 4.25 cm diameter) on the
bottom. Adult bed bugs destined to be used as donors were
introduced in Petri dishes containing the dust for 10 minutes. After
the 10-minute acquisition period, the donor (exposed) individuals
were then introduced into clean Petri dishes (Gelmon SciencesH;
5.0 cm diameter) along with the recipient (unexposed) bugs. Each
Petri dish contained a filter paper (Fisher ScientificH; 4.25 cm
diameter) folded in quarters serving as a harborage for the bed
bugs. Mortality was assessed every 24 h. Controls were comprised
of unexposed bed bugs only. Experiments were conducted at room
temperature (2362uC). At the end of the experiments digital
images of representative insects were taken with an Olympus DSX
500 microscope for visual confirmation of the transfer of dust.
Mortality among the receipient bed bugs was used as a measure of
horizontal transfer of the insecticide.
Horizontal transfer of DE 51 in C. lectularius through a
single exposed bed bug
This experiment was conducted to demonstrate if a single bed
bug exposed to DE 51 could transmit the dust to other unexposed
bed bugs by sharing a harborage.
a. Determination of LC
50
values. DE 51, red fluorescent
dye (20 mg, 15 mg, 10 mg and 5 mg each) and a mixture of DE
51 and dye containing Kthe amount of dye and DE 51 in a 1:1
ratio, for each concentration, were weighed in Petri dishes lined
with filter papers followed by the introduction of an adult bed bug
into each for a 10-minute acquisition period as described earlier.
The exposed bed bug was then introduced in a Petri dish (5.0 cm
diameter) containing 5 unexposed adult bed bugs. Petri dishes
were placed in a plastic box with a lid. There were 5 replicates of 6
bugs each. Controls consisted of adult bed bugs, none of which
were exposed to DE 51.
Mortality was assessed every 24 h for 10 days and was used as a
measure of transfer. Dye was used in the experiment for visual
confirmation of the transfer of DE 51 between exposed and
unexposed bed bugs. LC
50
values (concentrations needed to kill
50% of the bed bugs) were calculated for each time interval.
b. Determination of LT
50
values. Time to kill 50% of the
bed bugs was calculated for all groups as described in the previous
section.
Horizontal transfer of DE 51 to nymphs through exposed
adult bed bugs
This experiment was conducted to demonstrate if DE 51 can be
transferred from adult bed bugs to nymphs and was conducted the
same way as the transfer between adults except that 4 exposed
adult bed bugs were introduced into Petri dishes with 6 unexposed
first or second instar nymphs. There were four replicates of 10
insects each. LC
50
values were calculated for both adults and
nymphs at 24 and 48 h intervals.
Horizontal transfer of botanical insecticides through a
single exposed adult bed bug
This experiment was conducted to demonstrate if different
botanical insecticides could also be transferred like DE, therefore,
DE 51 was used as a positive control in this experiment.
Twenty mg samples of neem powder (containing 30%
azadirachtin as the active ingredient), ryania dust (containing
approx. 1% ryanodine as the active ingredient), rotenone garden
dust (containing 1% rotenone as the active ingredient) and DE 51
were weighed in Petri dishes lined with filter papers followed by
the introduction of an adult bed bug into each for a 10-minute
acquisition period as described earlier. One exposed adult bed bug
was then introduced in a Petri dish (5.0 cm diameter) containing 5
unexposed adult bed bugs. There were 4 replicates of 6 bugs each.
The control group consisted of 6 unexposed adult bed bugs.
Mortality was assessed at 48, 96 and 144 h.
Statistical analysis
LC
50
values along with their corresponding 95% confidence
intervals as well as LT
50
values were calculated via Probit analysis
using the EPA probit analysis program version 1.5. Four or five
concentrations were used for each treatment (n = 3–4 replicates of
6–10 insect each). Data for the horizontal transfer of insecticides
were analyzed by analysis of variance (ANOVA) using statistics
software [20]. Where significant Fvalues were found, Tukey’s
multiple comparison test was used to test for significant differences
between individual treatments.
Horizontal Transfer of Pesticides in Bed Bugs
PLOS ONE | www.plosone.org 2 September 2013 | Volume 8 | Issue 9 | e75626
Results
Horizontal transfer of DE 51 in C. lectularius through a
single exposed bed bug
a. Determination of LC
50
values. Mortality of the recipient
(unexposed) bed bugs placed with a DE 51- exposed bed bug was
minimal for the first 24 hours but increased markedly thereafter.
LC
50
values were 24.4 and 5.1 mg at 48 h and 216 h, respectively
(Table 1). Although the lowest LC
50
value was observed at 216 h,
it does not differ statistically from LC
50
values calculated at 120
and 144 h, based on their overlapping confidence intervals
(Table 1).
Mortality of the unexposed bed bugs placed with a dye +DE 51
mixture-exposed bed bug was minimal (22%) until 96 h but
increased markedly (55.6%) at 216 h. LC
50
values of the
unexposed bed bugs placed with a dye +DE 51 mixture-exposed
bed bug were 28.9 and 17.6 mg at 144 and 216 h, respectively
(Table 1).
Mortality was 10% in the unexposed bed bugs placed with a
dye-only exposed bed bug (Table 1) at 216 h and no mortality
Figure 1. General experimental procedure.
doi:10.1371/journal.pone.0075626.g001
Table 1. Horizontal transfer of diatomaceous earth, DE 51 in
C. lectularius adults through a single exposed bed bug.
LC
50*
(95% C.I)
Time (h)
DE 51 dye
+
DE 51
48 .20.0 2
72 13.9 (9.5–30.2) 2
96 13.0 (7.5–43.9) 2
120 8.7 (4.6–12.1) 2
144 6.4 (2.8–8.7) .20.0
216 5.1 (1.7–7.3) 17.6 (11.1–5134.2)
*LC
50
values (concentration causing 50% mortality; in mg) were calculated
based on 4 concentrations (5–20 mg) for each sample when an exposed bed
bug was placed with 5 unexposed adult bed bugs.
2not calculated due to low mortality
There were 5 replicates of 6 bugs each. Exposed bed bug was excluded from
the data (N = 25)
LC
50
values for the dye were not calculated due to low mortality; there was no
mortality in the control group.
doi:10.1371/journal.pone.0075626.t001
Figure 2. Adult bed bug, ventral view, showing accumulation
of red dye particles around the bases of legs and distal tip of
the abdomen.
doi:10.1371/journal.pone.0075626.g002
Horizontal Transfer of Pesticides in Bed Bugs
PLOS ONE | www.plosone.org 3 September 2013 | Volume 8 | Issue 9 | e75626
occurred in the control group. The red fluorescent dye was visibly
transferred from the single exposed bed bug to all others (Figure 2).
b. Determination of LT
50
values. Mortality of the unex-
posed bed bugs placed with DE 51- exposed bed bugs at different
concentrations was calculated at different time intervals (24–
216 h) as reported in the previous section and LT
50
values (time to
kill 50%) were calculated for each concentration. LT
50
values
along with 95% confidence intervals for the unexposed bed bugs
placed with a DE 51- exposed bed bug ranged from 202.3 h
(133.7–698.8) or 8.4 days at a concentration of 5 mg to 42.2 h
(28.1–55.2) or 1.7 days at a concentration of 20 mg (Figures 3 and
4). The LT
50
value for the unexposed adults placed with 20 mg
DE 51-exposed bed bug was significantly different from all others
except for the LT
50
value at 15 mg. A plot of LT
50
values versus
different concentrations of DE 51 indicated a strong dose response
(R
2
= 0.9986) (Figure 4).
The LT
50
value for unexposed bed bugs placed with a bed bug
exposed to a 20 mg mixture of dye +DE 51 (1:1) was 201.3 h
(147.5–393.4) (8.4 days) (Figure 3). Mortality of the bed bugs
placed with other concentrations of the mixture was too low (22–
44.4% at 216 h) to calculate LT
50
values. There was 13%
mortality in the dye-only group and no mortality in the controls
(Figure 3).
Horizontal transfer of DE 51 to nymphs through exposed
adult bed bugs
Mortality was high in nymphs at 24 h when DE 51-exposed
adult bed bugs were placed with the unexposed nymphs. LC
50
values for unexposed nymphs (8.1 mg) and unexposed adults
(6.4 mg) did not differ significantly based on their overlapping
confidence intervals (Table 2). Since 24 h mortality of the nymphs
(55.6–94.4%) placed with 10–20 mg of DE 51- exposed adult bed
bugs respectively and the mortality of the exposed bed bugs (66.7–
100%) was greater than 50%, LT
50
values were not calculated.
High mortality rates within the first 24 h indicates that time to kill
50% of the nymphs and adults falls within the first 24 h.
Mortality was considerably higher in nymphs when placed with
dye +DE 51-exposed adults.
24 h LC
50
value for nymphs was 19.9 mg, whereas there was no
mortality in the adults (Table 2). There was a significant decline in
the LC
50
values of the nymphs at 48 h. LC
50
values of the nymphs
(8.9 mg) and adults (11.8 mg) at 48 h did not differ significantly
based on their overlapping confidence intervals (Table 2).
There was no mortality in the adults or nymphs when dye-
exposed adults were placed with nymphs. There was also no
mortality in the control group (unexposed adults and nymphs)
(Table 2).
Horizontal transfer of botanical insecticides through a
single exposed adult bed bug
There was transfer of dust and botanical insecticides from a
single exposed adult bed bug to unexposed adults (Figure 5).
Mortality of the unexposed bed bugs was 83.3% (68.3), 41.7%
(68.3), 24.9% (66.8) and 29.2% (67.9) when placed with a bed
bug exposed to 20 mg of DE 51, neem, ryania, and rotenone,
respectively at 48 h (Figure 5). A one-way ANOVA on the
Figure 3. Horizontal transfer of DE 51 in
C. lectularius
through
an exposed bug; one bed bug exposed to 20 mg of DE 51 or a
mixture of DE 51 and the dye (10 mg each) or dye (20 mg)
alone was placed with 5 unexposed bed bugs; LT
50
= time
required to kill 50% of the bugs. There was no mortality in the
control group. There were 5 replicates of 6 bugs each. Exposed bed bug
was excluded from the data; N = 25.
doi:10.1371/journal.pone.0075626.g003
Figure 4. LT
50
values of unexposed
C. lectularius
when placed
with a bed bug exposed to different concentrations of DE 51
(5, 10, 15 and 20 mg). There were 5 replicates of 6 bugs each.
Exposed bed bug was excluded from the data; N = 25.
doi:10.1371/journal.pone.0075626.g004
Table 2. Horizontal transfer of diatomaceous earth, DE 51 to
nymphs through exposed adult bed bugs.
LC
50*
(95% C.I)
Treatments Stage
24 h 48 h
dye nymph – –
adult – –
DE 51 nymph 8.1 (5.7–10.3)
+
adult 6.4 (2.9–8.7)
+
dye+DE 51 nymph 19.9 (15.0–46.1) 8.9 (7.1–10.6)
adult – 11.8 (8.9–15.5)
*LC
50
values (concentration causing 50% mortality; in mg) were calculated
based on 4 concentrations (5–20 mg) for each sample when four exposed adult
bed bugs were placed with six unexposed nymphs (1
st
–2
nd
instar)
2not calculated due to low mortality
+not calculated due to high mortality
There were 4 replicates of 10 bugs each. Mortality is based on a total of 24
nymphs and 16 adult bed bugs.
doi:10.1371/journal.pone.0075626.t002
Horizontal Transfer of Pesticides in Bed Bugs
PLOS ONE | www.plosone.org 4 September 2013 | Volume 8 | Issue 9 | e75626
mortalities of different groups produced a significant F value
(F
4,19
= 18.8; P#0.05). Mortalities of unexposed bed bugs placed
with bed bugs exposed to DE 51 and neem were significantly
greater than the control (Turkey’s test: P#0.05). There was no
mortality in the control group.
Mortalities of the unexposed bed bugs at 96 h were greater than
48 h mortalities in all treatments except for the control (Figure 5).
Mortalities of all groups except for ryania were significantly
greater than the control (One-Way ANOVA; F
4,19
= 23.5;
Turkey’s test; P#0.05).
Likewise, 144 h mortalities of the unexposed bed bugs were
greater than those at 96 h (Figure 5). At 144 h, mortalities of all
groups were significantly greater than the control (One-Way
ANOVA; F
4,19
= 49.6; Turkey’s test; P#0.05). Mortality was ,5%
in the control group.
Discussion
Our results have demonstrated that bed bugs contaminated with
DE dust or botanicals caused mortality in other bed bugs while
sharing a harborage through contact. Transfer of dust occurred
not only between adults but also between adults and nymphs. The
amount of dust the bed bug was exposed to prior to being placed
with unexposed bed bugs seemed to play a major role in the time
of transfer and the resulting mortality, as demonstrated by the
concentration-response data. Use of a fluorescent dust provided
visual confirmation that contaminated bed bugs transfer dust to
unexposed bugs (Figure 2) in harbourage, confirming that bed
bugs do not need to be in direct contact with pesticide residues to
be affected. This result is important because bedbugs live in hard-
to-reach places (cracks, crevices, picture frames, books, furniture)
and as such interaction between the members of the colony can be
exploited for delivery and dissemination of control products.
Horizontal transfer of insecticides in bed bugs is likely facilitated
by their gregarious behavior, promoting contact between members
and allowing for the rapid transfer of materials (dust or
insecticides) from exposed to unexposed ones. Recent studies have
demonstrated that aggregation of bed bugs is mediated by a
combination of airborne [21] and contact pheromones [22], [23].
An airborne aggregation pheromone, composed of several short-
chain aldehydes and monoterpenes occurring in the exoskeleton of
immature bed bugs, has recently been shown to stimulate
aggregation of adult and immature bed bugs in harborages when
bugs are not foraging for hosts (e.g. during the photo phase). The
chemical environment associated with a refuge (i.e., the presence
of aggregation signals) has also been shown to be important for
inducing aggregation in other blood feeding bugs such as a kissing
bug [21]. In addition to pheromones, bed bugs are also known to
be attracted to their feces and exuvia [22], [24].
Horizontal transfer of insecticide occurs when foragers contact
or ingest an insecticide, return to the aggregation or nest, and
translocate the insecticide to other members as well as the refuge
and its vicinity. The crowded refuge conditions might be partially
responsible for more frequent contacts with the insecticide-
contaminated corpses, exuvium, faeces or parts of the contami-
nated substrate. Knowing that all stages of bed bugs (adults and
immature) typically emerge from their aggregations at night to
feed and return to their harborage before dawn [25], (personal
observation) an effective approach to contaminate bed bugs during
foraging would involve placing a toxicant in close proximity to
their aggregations. This strategy has been very successful in social
insects such as termites and ants as well as cockroaches, where a
bait is offered to the foragers who in turn deliver the insecticide to
the sedentary stages of the colony through contact, trophallaxis,
coprophagy, and necrophagy [26]. Since aggregations are often
located in inaccessible deep crevices and wall voids, a rapid
dispersal of the members is required to increase contact with the
toxicant. One way of achieving this is to augment these agents
such as diatomaceous earth with the addition of bed bug alarm
pheromones (E)-2-hexenal, (E)-2-octenal, and a (E)-2-hexenal:(E)-
2-octenal blend) as has been demonstrated previously [27].
Addition of alarm pheromone to diatomaceous earth prompted
a frenzied, rapid dispersal reaction in bed bugs, thereby promoting
contacts with the toxicant. Elevating temperature can also be an
effective strategy to increase insect movement and thereby
increasing contact with a desiccant [28].
We have demonstrated horizontal transfer of DE 51 and other
insecticides in our study. There was a progressive decrease in the
LC
50
values of DE 51 with time. Increased transfer of dust with
time might have resulted from greater contacts between the bed
bugs as well as with the contaminated substrate due to increased
bed bug movement. Physiological conditions such as hunger or
mating status are believed to be the driving forces initiating
movement in bed bugs [29]. Since bed bugs were not fed during
the course of the experiment, it is believed that search efficiency
for the host may have increased with time. Time to kill 50% of the
bed bugs was directly proportional to the amount of DE 51 the
donor bed bug was exposed to. It took almost 4.8 times longer to
kill recipient bed bugs when placed with a donor bed bug with
prior exposure to 5 mg of DE 51 as opposed to 20 mg of dust.
Horizontal transfer of the dust was not affected by the life stage
of the recipients. LC
50
values of the donors and recipients were not
significantly different from each other based on overlapping
confidence intervals. Comparison of the mortality data (Tables 1
and 2) demonstrated that although a mixture of dye and dust (1:1)
was not effective in transfer between adult bed bugs regardless of
the number of donors exposed, transfer was very effective between
the donor adults and the recipient nymphs (Table 2) even in the
first 24 h of exposure. However, the mixture was equitoxic to both
the donors and recipients at 48 h (Table 2).
Our study has demonstrated an effective horizontal transfer of
not only diatomaceous earth but also other dust-formulated
botanical insecticides with different modes of action including
Figure 5. Horizontal transfer of DE 51, neem, ryania and
rotenone in
C. lectularius
through an exposed bug; N = 4
replicates of 6 insects each; one bed bug with prior exposure
to 20 mg of DE 51, neem, ryania and rotenone was placed with
5 untreated bugs; control insects were not exposed to the
dust. There were 4 replicates of 6 bugs each. Exposed bed bug was
excluded from the data (N = 20). ** Significantly different from all
treatments including control. *Significantly different from control.
doi:10.1371/journal.pone.0075626.g005
Horizontal Transfer of Pesticides in Bed Bugs
PLOS ONE | www.plosone.org 5 September 2013 | Volume 8 | Issue 9 | e75626
neem, ryania and rotenone in C. lectularius. A significantly higher
transfer of DE 51 compared with other insecticides may be based
on its mode of action. Diatomaceous earth (DE) is usually regarded
as an abrasive that scratches the cuticular surface, absorbs
epicuticular wax and causes death through desiccation [30]
Diatomaceous earth-based insecticides are finding increased use
as stored commodity protectants, due to their nontoxic effects
against humans and animals [30]. They have also been shown to
be active against ticks [31], cowpea weevils [32], cockroaches,
silverfish [33], ants [34] and numerous pests of grain [35].
We have demonstrated strong toxic effects of DE 51 against the
bed bug, C. lectularius, in laboratory bioassays (24 h LC
50
and LC
95
values = 0.24 and 0.95 mg respectively) by directly exposing them
to the dust (Akhtar and Isman, unpublished data). LT
50
values for
bed bugs directly exposed to 2 and 20 mg of DE 51, viz. 10 and
5 h respectively (Akhtar and Isman, unpublished data), indicate
that at least 50% of the donor bed bugs will have more than
sufficient time to reach a harborage for the effective transfer of
toxin to other unexposed members of the colony before dying. The
time to kill is inversely related to the amount of insecticide used, so
there is a trade-off; selection of an intermediate dose of the
insecticide will cause less mortality but would permit more bed
bugs sufficient time to locate a harborage and transfer the toxin
leading to secondary and tertiary mortality of conspecifics. Given
that bed bugs stay in close contact with each other and release
aggregation pheromones to help relocate their harborage after a
blood meal [36], we believe that a contaminated bed bug will be
well able to reach a harborage within several feet of the source.
Although, auto-dissemination of Beauvaria bassiana conidia (a
microbial insecticide) via contact with contaminated individuals
has been reported previously in bed bugs [37] there are no
published studies on the horizontal transfer of dust and other
botanical insecticides in bed bugs to our knowledge. Taking
advantage of insect behaviour for designing novel control
strategies can be an excellent approach to deal with bed bug
infestations. Therefore development of different delivery systems
allowing for maximum contact of bed bugs with the dust or other
botanical insecticides will increase their impact through horizontal
transfer facilitated by gregarious behavior of bed bugs. Future
studies should focus on investigating this phenomenon in a field
setting and against insecticide-resistant strains.
Acknowledgments
We thank JP Textile Ltd., Vancouver, BC, for providing adult bed bugs
and the sample of DE 51.
Author Contributions
Conceived and designed the experiments: YA MBI. Performed the
experiments: YA. Analyzed the data: YA. Contributed reagents/materi-
als/analysis tools: YA. Wrote the paper: YA MBI.
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